Internal error reported on Roxie at closedown

Changes related to keeping channel information in roxie packet headers have introduced an issue at closedown of a one-shot roxie when processing the "sendUnloadMessage" message:

00000004 USR 2020-11-24 16:44:06.076 39 39 "ERROR: 1413: ccdmain.cpp(1235) : Internal Error at ccdqueue.cpp(2768)"

It's debatable whether this is worth sending in one-shot mode but that's a different issue!